This invention relates to high capacity D.C. battery backup systems and more particularly to the automatic measurement of battery cell capacity under actual load conditions to automatically activate an alarm system when the battery backup is incapable, or expected to be incapable, of providing uninterrupted service for a predetermined period of time if the main power source fails.
High capacity secondary or backup batteries are used in stationery applications such as at remote telephone equipment sites, electric utility substations, and industrial plants to perform vital functions of circuit breaker tripping and automatic switching, to provide for the orderly shut-down of generating units in an emergency, including the starting of emergency diesel-generators, and to power other similar equipment. These battery systems are commonly used to provide backup power in case there is a failure of a commercial A.C. power grid.
Some applications require a longer-term delivery of battery energy. These include emergency lighting for plants and hospitals and computer and communications equipment. Batteries must be ready to deliver their stored energy on demand to provide the needed uninterrupted service for a given period of time. Another typical application for an uninterruptible power supply is between a power grid and a large computer system. This application is used by financial, communication, manufacturing and other commercial industries. If the high capacity battery system is taken "off-line" for any reason, the necessary protection against power outages is lost at the time the battery system is not connected.
In summary, battery backup systems must be monitored and tested on a regular basis to ensure that sufficient back up energy from a D.C. power system is always available. Although batteries are reliable, any failure to supply the required energy would often result in serious or disastrous consequences.
The monitoring of battery backup systems to prevent said disastrous consequences is particularly needed with regard to telecommunications equipment. The battery backup system used with said equipment is deployed in large central office facilities to provide telephone and other services to both public and private business service customers. In order to provide services to rural and concentrated areas, telecommunications equipment is deployed in small facilities such as cabinets and controlled environmental enclosures. All equipment is powered primarily from A.C. provided by the local utility company. The primary A.C. power is backed up by a battery plant system which provides the D.C. power needed to operate the equipment. The backup system maintains the highly reliable telephone service used with said equipment.
Typically, a large industrial battery backup system includes a string or a plurality of parallel strings of serially connected rechargeable battery cells and a charger connected to the A.C. commercial power grid for maintaining the charge on the battery cells. Many of these battery backup systems, called "uninterruptible power supplies", are configured such that the load is never aware of any failure of the power grid because the battery system immediately supplies the necessary energy upon failure of the A.C. power grid.
Electric utility installations generally consist of 24, 60, or 120 individual 2-volt cells, which comprise a plurality of battery cells, connected together to provide 48, 120 or 240 volts D.C. The normal, steady-state load of sensors, indicating lights, relay coils, and electronic apparatus are supplied by a charger connected to the battery terminals. The charger also maintains the battery in a fully-charged state (i.e. electrolyte specific gravity of 1.230 for a typical lead-acid battery). This is known as a float-charge operation. The normal load applied to these batteries is complex and varies with time. It often is not possible to disconnect these loads for test purposes without interrupting the main circuits they control.
The battery terminal voltage or specific gravity of each cell making up the battery is an indicator which has been used to determine battery state-of-charge. It has been standard industry practice to take periodic specific gravity measurements and to conduct visual and other checks to determine battery state-of-charge. More and more systems are employing sealed cells where the measurement of specific gravity is not practical, however, specific gravity readings do not entirely indicate a battery system's ability to supply power. For example, the specific gravity for each cell in a battery may indicate a fully-charged ready state, but a high impedance in one intercell connection can prevent the battery from functioning as intended. It is therefore necessary to determine if a high impedance inter connection exists between one or more cells of a battery plant in determining whether the battery system can function as intended.
Load discharge tests can also be used to prove a battery system's ability to perform. A discharge test is specified by nuclear regulatory commission regulations 0123 (BWR) and 0452 (PWR) for each critical battery of a nuclear generating unit. These tests must be performed each time a unit is refueled. Each discharge reduces the remaining service life of a battery and may cause deterioration which can impair the battery's ability to function next time in an on-line situation. A low discharge test result may indicate trouble, but it will oftentimes not locate it.
The most accurate method of determining the actual capacity of a battery system is load testing or internal impedance measurements. Load testing and internal impedance measurements require on-site testing and evaluation by maintenance personnel or the collection of large amounts of historical data which are labor-intensive, require management coordination and can delay problem reporting.
While there are several methods and processes used to measure battery plant condition, these methods and processes have drawbacks. The measurement of the open circuit voltage is one method, but requires disconnection from the system for 24 hours. Thus, there is no backup power supply for 24 hours. The battery cell condition evaluation methods, as noted above, require substantial labor and the collection of large amounts of historical data. The measurements are taken by technicians on site and are thus labor intensive and costly. The measurement of the float voltage is also not a good method because internal impedances of the cells can rise over time.
It is thus important to have in-service indication that a battery is able to deliver its stored electrochemical energy when required, and also a means for locating an abnormal condition in one or more batteries while the battery system remains in service. Although routine visual inspections, specific gravity readings, voltage readings and periodic equalizing charge are all presently used in the prior art to keep a battery system ready to perform, they each have drawbacks which have accentuated the need for a fully automatic system which ensures that a battery system is ready to perform in the event A.C. power is interrupted. This automatic system should be a simple, reliable method of determining critical battery capacity and reporting an alarm message to avoid major failures or service outages. The below noted prior art has attempted to meet, but has not met, these needs.
U.S. Pat. No. 5,281,920 discloses an impedance measurement of battery cells within a battery system comprising one string of battery cells. The measurement is accomplished by dividing each of the strings into two portions. The loading current is only imposed on a portion of one of the strings at any given time. Battery cell voltage measurements are made only within the string portion.
U.S. Pat. No. 4,697,134 discloses a testing device measuring the impedance of secondary cells that form a battery system while the battery system is in a float charged condition and is connected to an active electrical load. The impedance measurement is made at a frequency selected to be different from those frequencies otherwise present in the charger load circuit. The device monitors the battery for a change in impedance that can signal a developing defect in one or more cells or connections that can prevent the battery from delivering stored energy to the load. The testing device is also used to compare the impedance of individual cells and connections to locate faulty components.
U.S. Pat. No. 5,321,626 teaches a system of battery cells and batteries monitored via sensing probes arranged in assemblies providing digital output signals which indicate sensed physical parameters. Data strings are provided to the probes and are modified to reflect present values of sensed parameters. The resultant data strings are processed and compared to store historical data to monitor and forecast performance of a system of connected batteries.
U.S. Pat. No. 4,743,855 addresses a method of measuring the state of discharge of a battery. The method consists in measuring a first internal impedance of the battery at a first frequency and a second internal impedance of the battery at a second frequency. Then a determination is made of the difference between the internal impedances which is representative of the state of discharge of the battery. This method provides an accurate way to check whether the batteries are in good working order.
Based on the drawbacks to prior art systems as noted above, it is an object of the present invention to detect changes in the overall capacity of a multicell secondary storage battery, including inter-cell connectors, by the continuous monitoring of the terminal to terminal voltage and battery string temperature while the battery string is connected to an active load.
It is also an object of the present invention to locate a defect from an individual cell or multiple cells of a multicell secondary storage battery, including during periods when the battery is connected to an active electrical load.
It is an additional object of the invention to obtain an automatic and accurate indication of a backup battery plant's capability to sustain the operation of equipment under actual environmental conditions without interrupting the D.C. power to the equipment so that an alarm may be activated if the backup battery plant is incapable of providing the necessary power for extended periods of time in the event the primary A.C. power is interrupted.
It is still a further object of the present invention to provide a procedure for determining and predicting a secondary storage battery's ability to supply power to a load, including during testing periods when the storage battery is connected to an active load.
It is an object of the present invention to provide an automatic D.C. battery plant alarm monitoring system, which minimizes the cost and time of labor in evaluating the performance and/or capability of the battery plant backup system.
It is another object of the present invention to activate an alarm when the backup battery system is incapable of supplying the required energy when the A.C. power grid fails. It is a further object of the present invention to provide an automatic alarm if the backup system is incapable of handling the energy requirements of the load for an extended period of time.
It is a further object of the present invention to provide necessary alarms in case the equipment at the application site is not operating correctly or will not operate correctly upon activation for a given time period.